As is well known, a large number of image display apparatuses are used as screen display apparatuses for a television receiver, computer, and the like. Among those, a passive light modulation type apparatus typified by a liquid crystal display apparatus, displays images on a passive light modulation part, which does not emit light by itself (liquid crystal panel, for example). Therefore, the screen of such apparatuses look dim as compared with display apparatuses of a light-emitting type such as CRTs. To cope with this, passive light modulation type image display apparatuses are generally provided with a light source (backlight, for example) which emits light from the rear side of a passive light modulation part therein to increase visual brightness of a display screen. Accordingly, brightness of the light source can be adjusted in addition to general contrast adjustment, thereby improving visibility of displayed images.
The levels of contrast and light source are basically adjusted manually by a user and fixedly set. In the recent years, however, to make the images more visible, various methods have been suggested to achieve dynamic adjustment of contrast and intensity of a light source (hereinafter, referred to as light adjustment) according to an input video signal that varies with time.
Such conventional methods for dynamically adjusting contrast and light source are exemplarily disclosed in Japanese Patent Laid-Open Publication Nos. 5-127608 and 8-201812 titled “liquid crystal display apparatus”. In the conventional adjustment methods disclosed in these publications, a maximum brightness level (MAX) and a minimum brightness level (MIN) of an input video signal are detected. When a difference between the maximum brightness level and the minimum brightness level is large, contrast is reduced, and is increased when the difference is small. Further, in the conventional adjustment methods, an average brightness level (APL) of the input video signal is detected. When the average brightness level is higher than a predetermined reference brightness level, brightness of the light source is reduced, and is increased when lower. As such, the conventional adjustment methods aim to always achieve constant display brightness.
In the conventional adjustment methods disclosed in the above publications, however, contrast adjustment (i.e., signal amplitude control) and light source brightness adjustment are separately performed (that means there is no correlation between both adjustments). Accordingly, the above described conventional adjustment methods cannot provide sufficient effect in improvement of a sense of contrast.
The light source is mainly implemented by a fluorescence lamp in view of light-emitting efficiency. General characteristics of the fluorescence lamp are now described by referring to FIG. 27 to FIG. 29. FIG. 27 is a diagram exemplarily showing a characteristic of a general fluorescence lamp, that is, a characteristic of lamp temperature to light emitting-efficiency. FIG. 28 is a diagram exemplarily showing a characteristic of lamp tube current to lamp temperature. Note that FIG. 28 shows a case where the fluorescence lamp is used as a back lamp, and shows a characteristic that the lamp temperature becomes 65° C. at current i0. FIG. 29 is a diagram exemplarily showing a characteristic of lamp tube current to brightness.
First, as shown in FIG. 27, for the general fluorescence lamp in use, there exists a temperature at which its light-emitting efficiency reaches maximum due to vapor pressure of mercury inside the lamp tube (in the drawing, 65° C.). Next, as shown in FIG. 28, the general fluorescence lamp, due to heat produced by itself, shows such a relation that the lamp temperature is in proportion to the lamp tube current. According to the characteristics shown in FIG. 27 and FIG. 28, the efficiency of brightness adjustment (light-emitting efficiency) of the fluorescence lamp is resultantly declined in either case of the lamp tube current being larger or smaller than the current i0, as shown in FIG. 29.
If taking this into consideration, the conventional adjustment methods as disclosed in the above publications are inevitably required to utilize a linear part of the characteristic shown in FIG. 29. This is because the methods aim to achieve constant visual brightness (display brightness) by adjusting the intensity of the light source based on the detected average brightness level. Accordingly, in the conventional adjustment methods, the light source cannot be efficiently used (that is, the maximum brightness cannot be obtained).
Further, the life of the lamp used as the light source varies with lamp tube current and temperature. Therefore, in the conventional image display apparatuses that adjust the intensity of the light source brightness according to a video signal, such a problem has existed that, if the video signal is uneven in its characteristic, a lamp tube current (drive current) of a large value flows in the tube for a long time, thereby shortening the lamp life.
Therefore, a first object of the present invention is to provide an image display apparatus and an image display method capable of visually improving a sense of contrast without increasing power consumption of a light source by carrying out contrast adjustment (signal amplitude control) and brightness adjustment of the light source such that they correlate.
A second object of the present invention is to provide an image display apparatus and an image display method capable of dynamically and optimally adjusting the intensity of the light source according to an input video signal by utilizing a range in the vicinity of a characteristic to maximize the light-emitting efficiency of the light source.
Further, a third object of the present invention is to provide an image display apparatus and an image display method capable of dynamically and optically adjusting the intensity of the light source according to an input video signal while securing the life of the light source required as a product.